LM2673LD-ADJ National Semiconductor, LM2673LD-ADJ Datasheet - Page 13

• Manufacturer Part Number: LM2673LD-ADJ
• Description: IC REG SW ADJ 3A STP-DOWN 14LLP
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM2673LD-ADJ

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.2 ~ 37 V
• Current - Output: 3A
• Frequency - Switching: 260kHz
• Voltage - Input: 8 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-LLP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LM2673LD-ADJTR

Application Hints
rms current rating greater than 1.25A (1/2 I
using Table 2 for specific component characteristics the
following choices are suitable:
1 x 1000µF/63V Sanyo MV-GX (code C14)
1 x 820µF/63V Nichicon PL (code C24)
1 x 560µF/50V Panasonic HFQ (code C13)
Step 6: From Table 5 a 3A or more Schottky diode must be
selected. The 20V rated diodes are sufficient for the appli-
cation and for through-hole components two part types are
suitable:
1N5820
SR302
Step 7: A 0.01µF capacitor will be used for Cboost. For the
50mS softstart delay the following parameters are to be
used:
I
t
V
V
V
V
Using Vin max ensures that the softstart delay time will be at
least the desired 50mS.
Using the formula for Css a value of 0.148µF is determined
to be required. Use of a standard value 0.22µF capacitor will
produce more than sufficient softstart delay.
Step 8: Determine a value for R
current limit of at least 2.5A plus 50% or 3.75A.
Use a value of 10KΩ.
ADJUSTABLE OUTPUT DESIGN EXAMPLE
In this example it is desired to convert the voltage from a two
battery automotive power supply (voltage range of 20V to
28V, typical in large truck applications) to the 14.8VDC alter-
nator supply typically used to power electronic equipment
from single battery 12V vehicle systems. The load current
required is 2A maximum. It is also desired to implement the
power supply with all surface mount components. Softstart is
not required.
Step 1: Operating conditions are:
Vout = 14.8V
Vin max = 28V
Iload max = 2A
Step 2: Select an LM2673S-ADJ. To set the output voltage
to 14.9V two resistors need to be chosen (R1 and R2 in
Figure 2). For the adjustable device the output voltage is set
by the following relationship:
Where V
A recommended value to use for R1 is 1K. In this example
then R2 is determined to be:
SST
SS
SST
OUT
SCHOTTKY
IN
: 50mS
: 16V
: 3.7µA
: 0.63V
: 3.3V
FB
: 0.5V
is the feedback voltage of typically 1.21V.
(Continued)
ADJ
to provide a peak switch
load
max). Again
13
R2 = 11.23KΩ
The closest standard 1% tolerance value to use is 11.3KΩ
This will set the nominal output voltage to 14.88V which is
within 0.5% of the target value.
Step 3: To use the nomograph for the adjustable device,
Figure
Volt • microsecond constant (E • T expressed in V • µS) from
the following formula:
where V
switch which is R
be typically 0.15Ω x 2A or 0.3V and V
across the forward bisased Schottky diode, typically 0.5V.
The switching frequency of 260KHz is the nominal value to
use to estimate the ON time of the switch during which
energy is stored in the inductor.
For this example E • T is found to be:
Using Figure 6, the intersection of 27V • µS horizontally and
the 2A vertical line (I
inductor, should be used.
From Table 1, L38 in a surface mount component is available
from Pulse Engineering with part number PE-54038S.
Step 4: Use Table 6 to determine an output capacitor. With a
14.8V output the 12.5 to 15V row is used and with a 68µH
inductor there are three surface mount output capacitor so-
lutions. Table 2 provides the actual capacitor characteristics
based on the C Code number. Any of the following choices
can be used:
1 x 33µF/20V AVX TPS (code C6)
1 x 47µF/20V Sprague 594 (code C8)
1 x 47µF/20V Kemet T495 (code C8)
Important Note: When using the adjustable device in low
voltage applications (less than 3V output), if the nomograph,
Figure 6, selects an inductance of 22µH or less, Table 6 does
not provide an output capacitor solution. With these condi-
tions the number of output capacitors required for stable
operation becomes impractical. It is recommended to use
either a 33µH or 47µH inductor and the output capacitors
from Table 6.
Step 5: An input capacitor for this example will require at
least a 35V WV rating with an rms current rating of 1A (1/2
Iout max). From Table 2 it can be seen that C12, a 33µF/35V
capacitor from Sprague, has the required voltage/current
rating of the surface mount components.
Step 6: From Table 5 a 3A Schottky diode must be selected.
For surface mount diodes with a margin of safety on the
voltage rating one of five diodes can be used:
SK34
30BQ040
SAT
6,
is the voltage drop across the internal power
requires
ds(ON)
load
times I
a
max) indicates that L38 , a 68µH
calculation
load
. In this example this would
D
is the voltage drop
of
the
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inductor